2004 Envoy slt. XL 6 cylinder sometimes won't start, when I turn the key I'll here a click in center dash. I'll disconnect (-) battery cable or tap center console, or bang on sterring wheel, maybe after 5 or ten minutes ill start then run good all day 1 day later I'll not start again.

A very common issue on the Envoy and Trailblazer models. You need a new ignition switch to fix this problem.

What is happening is the contacts inside the ignition switch become worn and often times need a whack on the steering wheel or column to make contact. I've probably replaced a hundred of these for that same problem and other electrical issues.

If you plan to replace the switch yourself, it isn't too detailed of a job. You would need to remove the plastic trim covers around the column, and the ignition switch is right there below the key cylinder. Remove the connector and push the two tabs in on the side of the switch to remove it from the housing. Be sure to align the gear on the new switch as the same position on the old switch or you will have problems. You'll see what I mean if you take it apart.

Having an issue with my envoy starting. When I turn the key it sounds like it's trying to start then nothing, all the dash lights come on and stereo and I hear a clicking noise. There's also a message on my dash that pops up saying brake system failure. Could that be why it's not starting?

A brake system failure won't keep the engine from starting. (usually) When you turn the key to start your engine you aren't actually making the contact with your starter solenoid like cars used to be. Your key is now telling your computer to start the engine. The computer turns on the starter solenoid and cranks the engine over. If the engine doesn't see things happening the way it wants them to run it will stop cranking the engine and do nothing. Each time you put your key to the start position the computer will go over the start sequence again.

a low battery can cause problems where the computer may not be able to function properly. Security of anti-theft will cause the engine to crank and not start. A dash indicator will come on when the security isn't seeing the key it has been programmed to see at start up.